Liquid crystal compounds are widely used as a material for display elements. Most of display elements comprising a liquid crystal are used in a TN-type display system, and liquid crystal materials belonging to a nematic phase are used for this purpose.
Since the TN-type display system is a non-emissive display type, it has advantages such as freedom from occurrence of eyestrain and small power consumption, but on the other hand it has drawbacks such as slow response and disappearance of display at certain visual angle. In recent years, said system has been converted so that features in the form a a flat display could be utilized. In particular, quick response and wide visual angle have been demanded.
In order to meet the above demand, an attempt has been made to improve the liquid crystal material. However, the TN display system is considerably inferior to other emissive display element (e.g., electrouminescent display and plasma display) in the response time and width of visual angle. Therefore, development of a novel liquid crystal display system as an alternative of the TN type display system is indispensable to attain a combination of utilization of the features of a liquid crystal display element, such as non-emissive type and small powr consumption, with attainment to response characteristics comparable to those of the emissive display. As one of the attempts, N.A. Clark and S.T. Lagerwall have proposed a display system wherein an optical switching phenomenon of a ferroelectric liquid crystal is utilized [see Appl. Phys. Lett., Vol. 36, p. 899 (1980)].
The existence of a ferroelectric liquid crystal has been first published in 1975 by R.B. Mayer [see J. Phys., Vol. 36, p. 69 (1975)], and the ferroelectric liquid crystal belongs to a chiral smectic C phase, a chiral smectic I phase, a chiral smectic F phase, a chiral smectic G phase, and a chiral smectic H phase (hereinafter simply referred to as "S.sub.c * phase", "S.sub.I *", "S.sub.F *", "S.sub.G * phase", and "S.sub.H * phase", respectively) from the viewpoint of the structure of the liquid crystal.
Many characteristics are required of a ferroelectric liquid crystal material for use in a ferroelectric liquid crystal display element actually utilized in the system. These characteristics requirements cannot be met by use of a single compound, which makes it necessary to use a ferroelectric liquid crystal composition obtained by mixing several liquid crystal compounds or non.liquid crystal compounds.
In addition to the ferroelectric liquid crystal composition consisting of a ferroelectric liquid crystal compound only, Japanese Patent Application Laid-Open No. 195187/1986 discloses that a basic substance composed of a compound and a composition exhibiting a phase such as a nonchiral smectic C, F,G, H, or I phase (hereinafter abbreviated to a "phase such as S.sub.C ") is mixed with at least one compound exhibiting a ferroelectric liquid crystal phase to prepare a ferroelectric liquid crystal composition as a whole. Further, there is a report that a basic substance composed of a compound and a composition exhibiting a phase such as a nonchiral S.sub.C is mixed with at least one compound exhibiting optical activity but not exhibiting a ferroelectric liquid crystal phase to prepare a ferroelectric liquid crystal composition as a whole [see Mol. Cryst. Liq. Cryst., 89. 327 (1982)].
When all the above facts are put together, it can be understood that a ferroelectric liquid crystal composition can be formed by mixing a basic substance with at least one compound having optical activity independent of whether the compound exhibits a ferroelectric liquid crystal phase.
The above-described smectic liquid crystal mixture comprising a combination of a basic substance exhibiting at least one phase such as non-chiral S.sub.C with at least one phase such as S.sub.C will be hereinafter referred to as the "base Sm mixture".
The base Sm mixture is preferably a liquid crystal mixture exhibiting an S.sub.C phase in a wider temperature range including room temperature from a practical viewpoint. Several of phenylbenzole-based, shiff-based, phenylpyridine-based and 5-alkyl-2-(4-alkoxyphenyl)pyridine mixtures, etc. have been used as the component of the base Sm mixture. For example, Japanese Patent Application Laid-Open No. 291679/1986 and a pamphlet of PCT International Publication No. W086/06401 each discloses a ferroelectric liquid crystal comprising a mixture of 5-alkyl-2-(4-alkoxyphenyl)pyrimidine with an optically active compound. The former discloses that use of a ferroelectric ectic liquid crystal material comprising said pyrimidine derivative as a base Sm mixture enables the response time of a light switching element to be shortened. Further, Japanese Patent Application Laid-Open No. 291679/1986 discloses that a ferroelectric liquid crystal material comprising 5-alkyl-2-(4'-alkylbiphenyl-4)pyrimidine, the above-described 5-alkyl-2-(4'-alkoxyphenyl)pyrimidine and an optically active compound is also useful for improving the response characteristics.
However, comparison of the liquid crystal display with other display elements, such as emissive display, has revealed necessity of a further improvement in the response characteristics of the liquid crystal display.
The first object of the present invention is to provide a ferroelectric liquid crystal composition having quick response through a further improvement in the invention described in the above-described Japanese Patent Laid-Open No. 291679/1986.
The second object of the present invention is to provide a light switching element containing said ferroelectric liquid crystal composition.